FIG. 7 is a fragmentary sectional view of a semiconductor module. A semiconductor module 500 includes a radiator plate 51, a back electrode film 53 of an insulating substrate with a conductive pattern (hereinafter simply referred to as the insulating substrate 56) which is adhered to the radiator plate 51 with solder 52 between, a semiconductor chip 57 adhered to a conductive pattern 55 formed over the insulating substrate 56 with solder between, a bonding wire 59 which connects a surface electrode (not illustrated) of the semiconductor chip 57 and the conductive pattern 55, a wiring conductor 60 one end of which is connected to the conductive pattern 55, a lead-out terminal 61 to which the other end of the wiring conductor 60 is adhered, a resin case 62 to which the lead-out terminal 61 is adhered, and gel 63 with which the resin case 62 is filled. Clamp holes 64 are made in the radiator plate 51 of the semiconductor module 500 for fixing the radiator plate 51 onto a cooling fin (not illustrated). The above insulating substrate 56 includes the back electrode film 53, an insulating plate 54 which is a ceramic plate or the like, and the conductive pattern 55 formed over its surface. In many cases, the back electrode film 53 and the conductive pattern 55 are formed by the use of copper foil or the like.
FIG. 8 is a fragmentary sectional view of the semiconductor module fixed onto a cooling fin. The semiconductor module 500 is fixed onto a cooling fin 65 by inserting bolts or the like (not illustrated) into the clamp holes 64 made in a periphery of the radiator plate 51.
FIG. 9 is a schematic sectional view of a flat radiator plate to which two insulating substrates of the same shape are soldered. As illustrated in FIG. 7, each insulating substrate 56 is obtained by forming the back electrode film 53 on the back of the insulating plate 54, such as a ceramic plate, and forming the conductive pattern 55 on the front of the insulating plate 54. The coefficient of thermal expansion of the radiator plate 51 made of copper or aluminum is higher than that of each insulating substrate 56, so the radiator plate 51 to which the insulating substrates 56 are soldered curves due to a bimetal effect so that it will be convex on an insulating substrate (insulating substrates 56) side. That is to say, a curve 67 which is convex on the insulating substrate (insulating substrates 56) side is formed on a back 66 of the radiator plate 51. If the radiator plate 51 is attached in this state to the flat cooling fin 65, there will be a big gap 68 in the center of the radiator plate 51, resulting in a decrease in heat radiation efficiency. Measures to prevent this will now be described.
FIG. 10 is a schematic sectional view of a concave curve formed for canceling out the convex curve of the radiator plate illustrated in FIG. 9.
In order to cancel out the convex curve 67 illustrated in FIG. 9, a reverse curve 69 is formed in advance so that the center of the radiator plate 51 will be concave on the insulating substrate (insulating substrates 56) side (which is referred to as reverse curving or initial curving). This reverse curve 69 is formed rather sharp so that the radiator plate 51 will not become convex on the insulating substrate (insulating substrates 56) side even after soldering.
When the insulating substrates 56 are soldered to the radiator plate 51 by which the reverse curve 69 indicated by a dashed line is formed, the convex curve 67 illustrated in FIG. 9 is canceled out and a curve 70 which is concave on the insulating substrate (insulating substrates 56) side is formed. When the radiator plate 51 is attached in this state to the cooling fin 65 by the use of the clamp holes 64, the center of the radiator plate 51 (bottom 70a of the concave curve 70) touches the cooling fin 65, both ends of the radiator plate 51 are pressed against the cooling fin 65 with the bottom 70a as a supporting point, and the whole of the back 66 of the radiator plate 51 adheres to the cooling fin 65.
Furthermore, according to Japanese Laid-open Patent Publication No. 2007-88045, as illustrated in FIG. 11, if insulating substrates 71 and 72 of different shapes are soldered to a radiator plate 73, a sharp curve 74 including concave curves 74a, 74b, and 74c corresponding to the insulating substrates 71 and 72 and clearance between them, respectively, is formed in advance on the radiator plate 73. It is assumed that the sizes of the curves 74a, 74b, and 74c are R10, R20, and R30 respectively. When the insulating substrates 71 and 72 are soldered to the radiator plate 73, the sharp concave curve 74 becomes a gentle concave curve (not illustrated). The radiator plate 73 to which the insulating substrates 71 and 72 are soldered is attached to a cooling fin (not illustrated). As a result, there appears no gap between the radiator plate 73 and the cooling fin.
In addition, according to Japanese Laid-open Patent Publication No. 2008-91959 (not illustrated), when an insulating substrate is adhered to a radiator plate by the use of solder, a concave curve (reverse curve) is formed in advance on an insulating substrate side of the radiator plate and then the insulating substrate is soldered. By doing so, the radiator plate is kept in a reversely curved state (having a concave curve on the insulating substrate side) even after the soldering. By attaching the radiator plate in a reversely curved state to a cooling fin, there appears no gap between the radiator plate and the cooling fin.
In FIG. 10, if the insulating substrates 56 soldered to the radiator plate 51 have the same shape and are arranged left-right symmetrically, then the radiator plate 51 curves left-right symmetrically as a result of the soldering so that its center will be convex on the insulating substrate (insulating substrates 56) side. A bottom 69a of the concave curve 69 which cancels out this curve is positioned at the center of the radiator plate 51, so the concave curve 69 formed on the radiator plate 51 may be managed at a depth at the center of the radiator plate 51. As a result, management can be exercised easily. Next, a case where two insulating substrates of different shapes are soldered to a radiator plate will be described.
FIG. 12 is a schematic sectional view of a radiator plate to which two insulating substrates of different shapes are soldered. When insulating substrates 81 and 82 of different shapes are adhered to a flat radiator plate 83 by the use of solder 84, left-hand and right-hand portions of a convex curve 85 which appears on an insulating substrate (insulating substrates 81 and 82) side after the soldering differ in curvature. Therefore, in order to cancel out the difference in curvature, a concave curve 86 left-hand and right-hand portions of which differ in curvature, that is to say, curvatures of the left-hand and right-hand portions of which are R3 and R4, respectively, may be formed in advance on the radiator plate 83.
When the insulating substrates 81 and 82 are soldered to the radiator plate 83 having the curve 86, the radiator plate 83 is deformed into the radiator plate 83 having a concave curve 87 a bottom 87a of which is positioned under the insulating substrate 82.
With the method of forming the concave curve 86 in this way, a position at which the left-hand and right-hand portions of the curve 86 which differ in curvature, that is to say, the curvatures of which are R3 and R4, respectively, connect deviates from the center and becomes unclear. Accordingly, it is difficult to manage the curve 86.
Furthermore, the curve 86 the left-hand and right-hand portions of which differ in curvature, that is to say, the curvatures of the left-hand and right-hand portions of which are R3 and R4, respectively, may be formed on the radiator plate 83 for canceling out the convex curve 85 which appears as a result of soldering the insulating substrates 81 and 82 of different shapes. Accordingly, the management of the curve 86 is complex.
FIGS. 13A and 13B are schematic views of the radiator plate of FIG. 12 before and after being fixed onto a cooling fin. FIG. 13A is a schematic view of the radiator plate before being fixed onto the cooling fin. FIG. 13B is a schematic view of the radiator plate after being fixed onto the cooling fin. The reference sign 92 in FIGS. 13A and 13B indicates a clamp hole.
As illustrated in FIG. 13A, the bottom 87a of the concave curve 87 is positioned under the rigid insulating substrate 82 having high rigidity. As a result, stress concentrates in clearance 88 having low rigidity between the insulating substrates 81 and 82 and, as illustrated in FIG. 13B, a crack 89 tends to appear in the solder 84, or a crack (not illustrated) tends to appear in the insulating substrate 81 or 82 made of a ceramic. In addition, an end portion of the insulating substrate 82 is lifted up between the insulating substrates 81 and 82 and a gap tends to appear at a contact surface 91 between the radiator plate 83 and a cooling fin 90.
According to Japanese Laid-open Patent Publication No. 2007-88045, a curve corresponding to each of the insulating substrates 71 and 72 is formed without taking a curve of the entire radiator plate 73 into consideration. This may lead to considerable curve deformation of the entire radiator plate 73. If considerable initial curving is performed, the following problems, for example, tend to arise. A semiconductor chip mounted over the insulating substrate 71 or 72 at assembly time shifts, or a void appears in solder between the insulating substrate 71 or 72 and the radiator plate 73.
Furthermore, the insulating substrates 71 and 72 are simply placed over the radiator plate 73. Therefore, as described in FIGS. 13A and 13B, the bottom of the concave curve may be positioned under the rigid insulating substrate 72 having high rigidity. In that case, stress concentrates in the clearance 75 between the insulating substrates 71 and 72 having low rigidity and a crack tends to appear in solder by which the insulating substrate 72 is adhered to the radiator plate 73. In addition, an end portion of the insulating substrate 72 is lifted up in the clearance 75 between the insulating substrates 71 and 72 and a gap tends to appear between the radiator plate 73 and the cooling fin.
Furthermore, Japanese Laid-open Patent Publication No. 2008-91959 does not state that when the insulating substrates of different shapes are soldered to the radiator plate in order to prevent the above troubles, a bottom of the curve (initial curving) formed on the radiator plate before the soldering is positioned under clearance between the insulating substrate.